The reduction of nitrogen oxide (“NOx”) emissions from small industrial, commercial and electric utility boilers, gas turbines, and other lean burn stationary combustion sources continues to be a challenge. Primary measures, such as low NOx burners, flue gas recirculation, water injection, fuel staging or air staging, need to balance the impact on the efficiency and stability of combustion with the level of NOx reduction obtained and the risk of increases in other regulated pollutants, such as carbon monoxide or unburned hydrocarbons. Secondary measures, including selective non catalytic reduction (SNCR) and selective catalytic reduction (SCR), involve the injection of reagents, such as ammonia or urea, into the upper furnace or the flue gases to chemically convert NOx to elemental nitrogen.
Ammonia reagent is regulated as a hazardous substance, which has driven many end users to consider aqueous urea reagent as an alternative. While aqueous urea is not a hazardous substance, its application for NOx reduction requires additional design effort to make certain that the urea is fully gasified and does not leave intermediate solid by products that can foul surfaces and reduce chemical utilization.
In converting urea to ammonia for use in NOx reduction by SCR, the art generally teaches the injection of urea into a heated vaporizer or a flowing side stream of hot combustion gases and/or heated air to gasify the urea for subsequent distribution upstream of a NOx reduction catalyst.
The art teaches the bypass of combustion gases around a heat exchanger sections in a boiler to provide heat for gasification of urea to ammonia without unwanted byproducts. In that case some of the flue gas heat enthalpy used in generating steam and power is lost as the portion of hot gases extracted for urea decomposition does not pass through the heat exchanger but is bypassed around it and later returned to the main gas stream.
In other cases the art teaches the decomposition of urea to ammonia in a flowing side stream of gases that can utilize hot flue gas or supplemental firing of fuel to heat the gases or ambient air for urea decomposition on large-scale combustors. The art prescribes a residence time of 1-10 seconds for the decomposition of urea to ammonia. That does not address the needs of small combustion sources where small quantities of ammonia are needed and where smaller decomposition reactors and shorter residence times would be advantageous.
Yamaguchi, in U.S. Pat. No. 5,282,355, describes the prior art as using NOx free exhaust extracted by an exhaust gas recirculation fan to vaporize aqueous ammonia in a vaporizer from which it is injected into the flue upstream of a catalyst layer via an ammonia vapor pipe. He identifies aqueous urea as a precursor to aqueous ammonia which can also be vaporized by NOx free exhaust. For aqueous based solutions of ammonia, Yamaguchi suggests that 0.5-1.0 seconds are required to vaporize the ammonia solution and Yamaguchi does not address the time required for complete decomposition and gasification of an aqueous solution of urea.
Yamaguchi identifies concerns about the formation and deposition of solids from the reaction of ammonia with other exhaust gas species and so proposes using superheated steam from the boiler or other source to provide the heat to vaporize the aqueous ammonia or its precursor in a vaporizer. However, the use of steam from a boiler has a penalty associated with removing steam from the heat or power generation process and also with the cost of preparing de-mineralized boiler makeup water to replace the steam used in the vaporization of the aqueous ammonia or its precursor.
Peter-Hoblyn et al., in U.S. Pat. No. 5,809,774, describe the use of SCR for NOx reduction from lean burn engines in conjunction with fuel treatment using oil and water emulsions for a portion of the NOx reduction. Peter-Hoblyn et al. suggest that for SCR, especially at high loads, it is sometimes practical to introduce the aqueous solution of NOx reducing reagent into a slip stream (less than all, e.g., 5-25%) of the exhaust gases to achieve gasification of the reagent prior to mixing with the major or entire portion of exhaust gases.
In U.S. Pat. Nos. 5,968,464 and 6,203,770, Peter-Hoblyn et al. teach that the injection of aqueous urea into a pyrolysis chamber with droplets of under 500 micron, and preferably under 100 micron, will facilitate complete gasification of urea prior to introduction into the exhaust gases and allow close coupling of the pyrolysis chamber and SCR catalyst. The use of a return flow injector is proposed to cool the injector and prevent solids from plugging the injector. The pyrolysis chamber of Peter-Hoblyn et al. is described in the specification and shown in the drawings as a small heated chamber with discrete holes disposed in the primary exhaust stream or as a foraminous structure that allows aqueous urea that has been gasified to ammonia in the chamber to escape into the flue gases and flow across a downstream SCR catalyst.
Peter-Hoblyn et al., however, do not describe how to prevent plugging of the compact pyrolysis chamber with urea decomposition products, especially at higher urea injection rates. Additionally, it is difficult to see how complete gasification of urea is accomplished in the pyrolysis chamber described by Peter-Hoblyn et al. While the process of Peter-Hoblyn et al. may work for low urea injection rates on the order of 10-25 grams/minute as required for passenger car diesel engines, it is not apparent how this approach would scale up for higher injection rates of 50-1000 grams/minute or greater, as often required for small stationary combustion sources.
Cho et al., in U.S. Pat. No. 5,296,206, describe the prior art as teaching the use of a flue gas slip stream drawn by a blower into a vaporizer vessel where the flue gas mixes and vaporizes aqueous ammonia, and also describes the use of an electric heater to heat ambient air and mix it with aqueous ammonia in a vessel, thus vaporizing the aqueous ammonia. Cho et al. identify both aqueous ammonia and urea as known reducing agents. Cho et al. propose using a heat exchanger in the flue gas to transfer heat to a heat transfer medium, such as ambient air, which is heated to 400° F.-950° F. and used to vaporize aqueous ammonia that is sprayed with an air assisted injector into a vaporizer vessel and from which vaporized reagent is then injected into the flue gas across a catalyst. Cho et al. avoid the need for external electricity or steam for vaporization but do not describe how the temperature in the vaporizer will be maintained at low loads and low flue gas temperatures across the heat exchanger, especially with the cooling effect of the aqueous reagent and atomizing air injected into the vaporizer.
In U.S. Pat. Nos. 7,615,200 and 7,815,881 directed at large scale combustors, Lin et al. teach that a side stream can be generated by bypassing some portion of flue gases around a heat exchanger surface, such as an economizer, into which aqueous urea can be injected and gasified prior to forming a combined stream across a catalyst. In U.S. Pat. No. 7,815,881, Lin et al. describe the bypass flow as less than 10% of the combustion gases. Obviously the overall combustor efficiency would be negatively affected if this large quantity of flue gas were bypassed around a heat exchanger. Lin et al. teach that at high loads with high temperatures the bypass damper can be closed; however, at low loads with low gas temperatures Lin et al. do not describe how this large quantity of bypassed gas would efficiently be brought up to a temperature sufficient for urea gasification.
Sun et al., in U.S. Pat. Nos. 7,090,810 and 7,829,033, describe a process for reducing NOx from a large-scale combustor involving a side stream of gases or heated ambient air into which urea is injected for decomposition and then introducing the side stream into a primary stream for NOx reduction across a catalyst. Sun et al. specifically teach that residence times of 1-10 seconds are required to effectively evaporate the water and gasify the urea such that solid byproducts do not foul the distribution pipes, ammonia injection grid (“AIG”) or catalyst or heat transfer surfaces. Supplemental heat from a burner, steam coil heater or other source can be utilized. These patents are generally directed at large combustors and describe the need for sophisticated vessel design for the side stream using computational fluid dynamic (“CFD”) modeling techniques.
Fuel Tech Inc. has commercially marketed a system called the ULTRA™ process which generally uses a burner to decompose large quantities of urea to ammonia for large-scale combustors and a related product called ULTRA-5™ for smaller applications which uses an electric heater to heat ambient air for urea conversion. In many applications, a burner requires an additional permit to operate. The use of ambient temperature atomizing air for the air atomized injector of the Fuel Tech processes can represent as much as 8% of the overall air through the decomposition chamber. That cooler air combined with the cooling effect of introducing aqueous urea into the decomposition chamber can result in an outlet temperature from the decomposition chamber that is under 600° F. and well below the minimum 650° F.-700° F. outlet temperature range which Applicants have found to be desirable. That can lead to incomplete decomposition of urea and/or to the need for the longer residence times as proposed by Sun et al.
The Sun et al. patents cited above, assigned to Fuel Tech Inc., generally teach 1-10 seconds residence time for complete gasification of urea before introduction into the bulk gas stream. However, Applicants have discovered that by balancing gas flow through the decomposition duct, temperature in the duct, urea injection rate and urea spray quality, the residence time requirement for complete urea gasification can be reduced to under 1 second, which may be desirable in certain circumstances.
The marketplace has been looking for a simple, cost effective and reliable method of converting urea to ammonia on small combustion systems where only small quantities of aqueous reagent are required to be gasified. The prior art would lead one to believe that complex vaporizer systems, decomposition vessels designed with CFD, heat exchangers inserted in the flue gas, steam extraction from a boiler, high secondary power requirements to heat ambient air above the reagent decomposition temperature or large side stream ducts with bypass dampers and long residence times are required to reliably vaporize even small quantities of aqueous urea to generate ammonia gas for SCR.
To the contrary, however, the present invention provides a method and apparatus that controls the rate of gas flow through the decomposition duct, maintains temperature in the duct, precisely controls the urea injection rate as a function of combustor load, targets and maintains urea spray quality without additional ambient atomizing air and reduces the residence time requirement for evaporation and gasification to under 1 second while minimizing the need for external power.
Combined cycle gas turbines fired by natural gas or petroleum based fuels represent an efficient combustion system for generating power. Due to the high exhaust temperatures and large volume of combustion gases the gas turbine is often combined with a heat recovery steam generator (HRSG) and a steam turbine to provide additional electricity and improved system efficiency. Combustion turbines with the high combustion temperatures and high excess air also produce a significant quantity of nitrogen oxide emissions. Thus even with the low concentration of emissions as measured in parts per million (ppm), the mass of NOx emissions can be high from a combustion turbine. The art has explored the use of SCR on both combined cycle and simple cycle turbines to reduce the NOx emissions. Most of the SCR applications on either simple cycle or combined cycle turbines use ammonia as the reagent and require high temperature vaporizers to convert the aqueous ammonia to an ammonia gas for injection upstream of the SCR catalyst.
Anderson et al., in U.S. Pat. No. 5,555,718, teach a method of injecting a reagent, such as urea or ammonia, into the expanding transition section at the inlet of the HRSG that receives the exhaust from the turbine. Arranged in the expanding transition section are low profile injection pipes for injecting the reagent into the exhaust ahead of a catalytic reactor section. Anderson et al. do not address the fact that many combined cycle turbines have a carbon monoxide catalyst positioned in the high temperature inlet section of the HRSG to oxidize CO in the exhaust to CO2. Reducing agents, such as ammonia, that are injected upstream of the CO catalyst would be oxidized to NOx across the CO catalyst.
Buzanowski, in U.S. Patent Application Publication No. 2004/0057888 A1, now abandoned, teaches the use of a blower fan using hot air or exhaust gas to vaporize aqueous ammonia in a vaporizer to feed an ammonia injection grid positioned in the primary exhaust upstream of an SCR reactor for NOx reduction. A plurality of adjustable valves can be used to adjust the ammonia distribution rate from ammonia distribution pipes by adjusting the amount of hot carrier gas to each injection pipe or section of pipes.